1. Field of the Invention
The present invention relates to a thermoelectric module that is a temperature control device using Peltier effect, and particularly a thermoelectric module with improved heat-transfer efficiency, and a method of manufacturing the thermoelectric module.
2. Disclosure of the Prior Art
As shown in FIGS. 10A and 10B, a conventional thermoelectric module 1P has a structure comprising an arrangement of N-type semiconductor elements 21P and P-type semiconductor elements 22P, which are arranged in a matrix manner such that each of the N-type semiconductor elements 21P is disposed adjacent to the P-type semiconductor element 22P through a required space, upper electrodes 5P disposed on a top surface of the arrangement to connect between adjacent semiconductor elements 21P and 22P according to a first circuit pattern, lower electrodes 6P disposed on a bottom surface of the arrangement to connect between adjacent semiconductor elements 21P and 22P according to a second circuit pattern different from the first circuit pattern, and ceramic plates 8P such as sintered alumina plates bonded to the upper and lower electrodes 5P and 6P.
For example, when direct current is supplied to the thermoelectric module 1P, each of the upper electrodes 5P has the flow of electricity from N-type semiconductor element 21P to the P-type semiconductor element 22P, and on the other hand the each of the lower electrodes 6P has the flow of electricity from the P-type semiconductor element 22P to the N-type semiconductor element 21P. At this time, the upper electrodes 5P absorb heat from the surroundings through the ceramic plate 8P, and the lower electrodes 6P radiate heat to the surroundings through the ceramic plate 8P. Therefore, the thermoelectric module 1P works as a kind of heat pump for pumping heat from one side to the opposite side thereof, which is usually called Peltier effect. According to this principle, it is possible to use the thermoelectric module 1P as a temperature control device for electronic parts or circuit boards
As materials for the semiconductor elements 21P and 22P, Bi.sub.2 Te.sub.3 and Sb.sub.2 Te.sub.3 are widely used. Since these compounds are brittle materials, cracks or chippings of the semiconductor elements easily occur during a manufacturing process of the thermoelectric module, so that there is a problem that the yields of the semiconductor-element materials are low. This increases the production cost and reduces a degree of reliability of the thermoelectric module. In addition, the ceramic plates 8P are usually soldered to the upper and lower electrodes 5P and 6P by the use of a solder material 9P to maintain the structural stability of the thermoelectric module 1P. Since thermal stress occurs according to a difference of thermal expansion coefficient between the semiconductor-element materials and the ceramic-plate material, cracks may be generated in the ceramic plates or the semiconductor elements by the thermal stress.
Japanese Patent Early Publication [KOKAI] No. 10-51039 discloses a thermoelectric module 1R having flexibility and resistance to thermal stress. In this thermoelectric module 1R, adjacent semiconductor elements 21R and 22R are mechanically connected by a supporting member 3R such as a silicone-resin adhesive having electrical insulation and flexibility in place of brittle ceramic plates, as shown in FIG. 11. Due to the flexibility of the supporting member 3R, the thermoelectric module 1R can be fitted and bonded to a curved surface. In addition, silicone films 51R having electrical insulation are formed on upper and lower electrodes 5R and 6R of the thermoelectric module 1R.
On the other hand, Japanese Patent Early Publication [KOKAI] No. 9-293909 discloses a method of manufacturing a thermoelectric module 1S for the purpose of increasing the yields of semiconductor-element materials. In this method, as shown in FIG. 12A, a thermoelectric chip 10S having exposed surfaces of N-type and P-type semiconductor elements 21S and 22S on its top and bottom surfaces 11S, 12S is prepared by making a matrix arrangement of the semiconductor elements, and integrally molding the matrix arrangement with an electrical insulation resin 3S such as epoxy resins. Subsequently, as shown in FIG. 12B, metal films 4S are formed on the exposed surfaces of the semiconductor elements 21S, 22S and the insulation resin 3S to connect between adjacent semiconductor elements according to a first circuit pattern on the top surface and a second circuit pattern on the bottom surface of the thermoelectric chip 10S. Copper electrodes 5S are then formed on the metal films 4S by electroplating, as shown in FIG. 12C. Since the semiconductor elements 21S and 22S are reinforced with the insulation resin 3S in the thermoelectric chip 10S, it is possible to reduce the occurrence of cracks or chippings of the semiconductor elements and improve the yields of the semiconductor element materials.
By the way, in order to accurately control the temperature of articles such as electronic parts and circuit boards by the use of the thermoelectric module, it is necessary to improve heat-transfer efficiency between the thermoelectric module and the articles, while maintaining electrical insulation therebetween. The silicone films 51R formed on the electrodes 5R, 6R of the thermoelectric module 1R shown in FIG. 11 provide the electrical insulation. However, the heat-transfer efficiency of the silicone film 51R is much lower than that of conventional ceramic materials. Conventional organic resins are of 1/50th to 1/200th thermal conductivity of alumina ceramic. Therefore, this thermoelectric module is susceptible to improvement from the viewpoint of heat-transfer efficiency.
On the other hand, in the thermoelectric module 1S of Japanese Patent Early Publication [KOKAI] No. 9-293909, a grease material 51S having electrical insulation is applied on the top and bottom surfaces 11S, 12S of the thermoelectric chip 10S, as shown in FIG. 12D, and then heat-transfer plates 52S made of a metal material having excellent thermal conductivity such as aluminum or copper are put on the grease material 51S, as shown in FIG. 12E. In this case, there are problems that the thermal conductivity of the grease material 51S is poor, and the structural stability of the thermoelectric module 1S is low because the heat-transfer plates 52S are merely put on the thermoelectric chip 10S through the grease material 51S. In addition, when the thickness of the grease material 51S partially becomes small, a short circuit may be caused between the electrodes and the heat-transfer plate. Therefore, it is required to apply the grease material 51S having poor thermal conductivity on the thermoelectric chip 10S with a thickness sufficient to maintain the electrical insulation therebetween.